1. Field of the Invention
The present invention relates to an exhaust gas recirculation system for an internal combustion engine and, more particularly, to an exhaust gas recirculation control system which will be termed hereunder as "E.G.R. control system" for the simplification purpose.
2. Description of the Prior Art
An E.G.R. control system has been known which is of the type that comprises an E.G.R valve means including a first valve seat provided in an E.G.R. passage of an internal combustion engine, a first valve member cooperative with the first valve member to vary the exhaust gas flowing cross-section of the E.G.R. passage and a vacuum-operated actuator connected by a first pneumatic line to a first vacuum source provided by the engine, such as a port formed in the engine carburetor at a point just upstream of the idle position of the engine throttle valve, and a fixed restriction provided in the E.G.R. passage upstream of the first valve seat. The actuator is also operatively connected to the first valve member to move the same relative to the first valve seat for thereby varying the exhaust gas pressure in the E.G.R. passage between the fixed restriction and the first valve seat, so that the flow of the engine exhaust gases from the engine exhaust system through the E.G.R. passage back into the intake system is controlled by the E.G.R. valve means.
In order that the flow of the recirculated exhaust gases is so controlled as to be substantially in proportion to the flow of the engine intake air through the intake pipe, a pressure modulator has been used to modulate the port vacuum applied to the valve actuator so that the actuator is so operated as to vary the pressure difference across the fixed restriction in the E.G.R. passage in substantially proportion to the engine intake air flow through the intake pipe of the engine.
The prior art pressure modulator comprises a housing and a diaphragm assembly which includes three diaphragms which are mechanically connected together so that the diaphragms are moved in unison in response to a variation in the venturi vacuum and also in response to a variation in the exhaust gas pressure in the E.G.R. passage between the fixed restriction and the first valve seat. The diaphragm assembly cooperates with the housing to define therein first to fourth chambers. The first chamber is defined between the housing and the first diaphragm and is vented to the atmosphere and pneumatically connected to the first pneumatic line by a pressure modulating passage having an inner end surrounded by a second valve seat disposed in the first chamber. A second valve member is disposed in the first chamber and carried by the diaphragm assembly for movement toward and away from the second valve seat so that the flow of the atmospheric air from the first chamber through the pressure modulating passage into the first pneumatic line is varied in dependence on the position of the second valve member relative to the second valve seat. The second chamber is defined between the first and second diaphragms and connected to the carburetor venturi by a second pneumatic line. The third chamber is defined between the second and third diaphragms and is simply vented to the atmosphere. The fourth chamber is defined between the third diaphragm and the housing and is pneumatically connected to the E.G.R. passage between the fixed restriction and the first valve seat of the E.G.R. valve means.
The prior art pressure modulator is operative fairly well to control the supply of the vacuum to the E.G.R. valve actuator so that the recirculated exhaust gas flow through the E.G.R. passage from the exhaust pipe back into the intake pipe is substantially proportioned to the flow of the intake air through the carburetor venturi. The problem, however, is that the prior art pressure modulator is complicated in structure, large-sized and expensive. This is because of the use of three diaphragms to form the diaphragm assembly in the prior art pressure modulator.